This application claims the priority of German application 198 17 391.1, filed in Germany on Apr. 20, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a spark plug for an internal combustion engine and/or a sensor element for an ignition and combustion process in which at least one of the electrodes consists of two materials, with the electrodes being so designed that the distance from a first area of one of the electrodes to the other electrode is less than the distance from an additional area of the one of the electrodes to the other electrode, with the two areas consisting of different materials.
The structure of conventional spark plugs, used for example in transistor-coil ignition systems, is generally known. A high voltage on the order of 30 kV is produced at the electrodes of a spark plug and leads to a sparkover between the electrodes of the plug. The mixture in the combustion chamber of the engine is ignited by this sparkover so that the combustion process begins. The sparkover followed by the arc causes material to be removed from the electrodes (burn-off). Consequently, the electrode gap gradually increases so that as a result of this increase in the size of the gap, the voltage increases that is required to produce the sparkover. It is likewise known to use such spark plugs as ion current sensors for measuring and/or checking the ignition as well as the combustion process (see European Patent Document No. EP 0 699 870 A1).
To avoid the problems associated with burn-off it is known (see German Patent Document No. DE 41 28 392 C2) to make the electrodes of a spark plug from several materials. A carrier material has another material added to it that forms a first area of the electrode. The carrier material forms a second area of the electrode. The distance between the two first areas of the electrodes is smaller than the distance between the second areas. The first area consists of a noble metal such as platinum for example or a noble metal alloy such as platinum-iridium for example. It is also indicated that the second area consists of a nickel alloy which is known of itself and is used in the electrodes of spark plugs. The intent is for sparkover to take place in the first area of the electrodes and for the sparkover to be transferred to the second area during the arc phase. Since the removal of material from the noble metals is extremely small, in the course of a very long lifetime of the spark plug the distance between the first areas at which the sparkover takes place remains almost constant so that the voltage required to produce a sparkover is constant over a longer service life of the spark plug. The primary removal of material during the arc phase takes place in the second area.
Accordingly, a goal of the invention is to propose an improved spark plug and/or an improved sensor element.
This goal is achieved according to the invention by providing a spark plug or sensor element of the above described general type wherein at least one of the electrodes has a small radius of curvature in the first area at the point where the distance from the other electrode is minimal.
This small radius of curvature can be a point for example, corresponding to an idealized radius of curvature of zero. It is important in this connection that the shape of the surface strengthens the electrical field which can be described as essentially inversely proportional to the radius of curvature, so that sparkover is promoted. This is the case for a point for example or for a suitably curved surface. From the prior art it is only known in this connection to make the surfaces flat, in other words with an infinite radius of curvature.
It has been found that as a result of the slight removal of material from the first area, the geometric shape of the first area likewise remains almost unchanged so that the small radius of curvature and/or the point also is retained over a long service life without being worn away. As a result, because of the electrical field that forms as a function of the radius of curvature of the electrodes, the voltage required for sparkover can be reduced. This permits a simplification of the design of spark plugs because the insulating materials such as ceramic boiler scale or plastic insulation are at the limit of their performance at the voltages previously required for sparkover (on the order of 30 kV). Increasing the wall thicknesses of the insulating materials to improve the insulation is not readily possible because it can cause uncontrolled surface and volume discharges that destroy the materials.
In another solution according to the invention, a spark plug or sensor of the above described general type is provided, wherein the shape of the electrode is continuous at least at the transition between the two areas.
As a result, the transfer of the spark following sparkover in the arc phase from the first area to the second area is facilitated by contrast with an arrangement in which there is a discontinuity at the transition from the first to the second area.
An especially advantageous improvement to known spark plugs is achieved according to especially preferred embodiments by providing a spark plug or sensor of the above described general type, wherein both of the features referred to above are included, namely, wherein at least one of the electrodes has a small radius of curvature in the first area at the point where the distance from the other electrode is minimal, and wherein the shape of the surface of the electrode is continuous, at least in the transition between the two areas.
It has been found to be advantageous if, according to certain especially preferred embodiments, at least one of the two areas is composed of several materials.
In the design of a spark plug according to certain preferred embodiments of the invention, as a result of the geometric shape of the two areas, for example thickenings, narrowings, threads, or grooves, the electrode is prevented from falling apart at the point where the two areas join, even under alternating mechanical and thermal stress.
In the design of a spark plug according to certain preferred embodiments of the invention, the nature of the connection between the two areas, for example welding, soldering, or shrinking, the electrode is prevented from falling apart at the point where the two areas join, even under alternating mechanical and thermal stress.
As a result, problems are avoided that can occur for example as a result of different thermal conducting properties of the materials or of erosion of the carrier materials.
Certain preferred embodiments of the invention provide for an improvement of the preferable embodiment of the spark plugs described above and include a feature by which improvements can be achieved in the spark plugs known from the prior art. Accordingly, at least one of the electrodes has a material projection at a point that is opposite a point on the other electrode at which material losses can occur at the other electrode during operation.
As a result of this design, an auxiliary spark gap is advantageously produced at which material deposits are eliminated by occasional surface discharges. The spark plug is therefore not prone to sooting.
Of course, it is advantageous if not just one of the electrodes is designed in this way but if both electrodes are designed according to the embodiments described and claimed.
Even though the conditions, for the sake of understanding and clarity of presentation, both in conjunction with the claims and also in the description of the figures, are explained only with reference to the spark plug, it is evident from the relationship and especially the claims that the spark plug can also be used as a sensor element and that the part described as a spark plug can be used simply as a sensor element without the combustion process being triggered by the part.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.